Blood vessel occlusions are commonly treated by mechanically enhancing blood flow in the affected vessels, such as by employing a stent. Stents act as scaffolding, functioning to physically hold open and, if desired, to expand the wall of affected vessels. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. Local delivery of a therapeutic substance is a preferred method of treatment because the substance is concentrated at a specific site and thus smaller total levels of medication can be administered in comparison to systemic dosages that often produce adverse or even toxic side effects for the patient.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
A shortcoming of the above-described method of medicating a stent is the potential for coating defects and the lack of uniformity of the amount of composition material sprayed onto stents. While some coating defects can be minimized by adjusting the coating parameters, other defects occur due the shot to shot variation leading to excess composition being sprayed onto the stent. One cause of this shot to shot variation is the type of spray coater used. For example, a conventional EFD N1537 (EFD Inc. East Providence R.I.) spray coater uses a valve mechanism to dispense fluid and is most suitable for dispensing large amounts of composition (i.e., grams) and not small amounts (e.g., milligrams per spray cycle) as used in stent coating applications. Accordingly, conventional spray coaters tend to spray excess coating onto stents, which may stick to the stent, thereby leaving excess coating as clumps or pools on the struts or webbing between the struts.
Accordingly, a new nozzle for spraying coating is needed to minimize coating defects.